1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a copier, and a facsimile machine.
2. Description of the Related Art
A typical image forming apparatus forms an image in the following manner. That is, an electrostatic latent image is first formed on a photosensitive element, the latent image is then developed into a toner image with a toner, the toner image on the photosensitive element is then transferred onto an intermediate transfer body, and the toner image on the intermediate transfer body is then transferred onto a recording medium, such as a paper sheet, and the image is finally fixed onto the recording medium by application of heat. Generally, a roller is used to carry such an image or a recording medium. The image or the recording medium is carried by the rotation of the roller. To drive the roller to rotate, a drive source such as a motor is connected to the roller via a drive-force transmission mechanism such as a gear or a timing belt.
There has been developed a retracting mechanism capable of retracting an element when the element is not required to form an image, or to carry the image or a recording medium. For example, in the technology disclosed in Japanese Patent Publication No. 3512307, in a multi-color image forming apparatus, when a black image is to be formed, only the element required to form the black image is activated, i.e., elements that are not required for the formation of the black image are retracted. Moreover, in a manual sheet feeding device disclosed in Japanese Patent Application Laid-open No. 2006-089189, a bottom plate is configured to be retracted downward when the manual sheet feeding device is powered OFF, so that a user can easily handle a recording medium. The retracting mechanism typically includes a cam capable of converting a power transmission from a drive source into a reciprocating motion.
Generally, an external force is applied to the reciprocating member by a spring or the like so that the reciprocating member is held in either an operating position or a retracted position as a home position. When the reciprocating member held in the home position moves to some other position due to the rotation of the cam, a force exceeding the external force is applied to the reciprocating member. At this time, a load torque is exerted on a camshaft. Subsequently, when the reciprocating member moves back to the home position from the other position, the external force acts as a drive force to the cam, i.e., the drive force is applied to the cam via the reciprocating member by the action of the external force. At this time, an acceleration torque is exerted on the camshaft. In this manner, because of the reciprocating motion of the reciprocating member between the operating position and the retracted position, the load torque and the acceleration torque are alternately exerted on the camshaft. Such a variation between the load torque and the acceleration torque is referred to as a load variation.
The drive source is set to output a power exceeding the maximum load torque of those exerted on the camshaft. Therefore, even if an average torque of the load torque is identical to that of the acceleration torque, when the load variation between the load torque and the acceleration torque is large, it is necessary to use the drive source capable of outputting a power higher than the maximum load torque. One approach is to modify the shape of the cam or the reciprocating member in a manner that leads to reduction in the amount of the load variation. However, this approach could lead to a decrease in the performance reliability of the cam or the reciprocating member. Moreover, the size and the production costs of the apparatus may increase if the modification results in a complicated configuration of the cam or the reciprocating member.
For example, when a direct current (DC) brush motor without a rotation control function depending on the load variation is used as the drive source, an angular velocity of the DC brush motor varies in synchronization with the load variation. Namely, as the amount of the load variation increases, the angular velocity of the DC brush motor also increases. Therefore, in such a configuration that a mechanism controls the cam to be driven or stop driving by detecting a rotational position of the cam and a position of the reciprocating member, the angular velocity varies depending on a working position. Therefore, as the load variation is getting larger, it is necessary to detect the positions of the cam and the reciprocating member more precisely and to set a control value more accurately.
Furthermore, in a case of the drive source without the rotation control function, a rotation rate of the drive source varies, so that a running sound of the drive source increases and decreases depending on a cycle of the variation of the rotation rate. Thus, a user may feel the running sound as a harsh noise.
Moreover, as described above, in an area in which the acceleration torque is exerted on the camshaft while the cam makes one revolution around the shaft, an acceleration for accelerating the cam or the reciprocating member to move is generated depending on a degree of the acceleration torque or the external force. When the reciprocating member moves back to the home position, the reciprocating member may make a relatively loud impact sound due to the acceleration. Thus, the user may feel the impact sound as a harsh noise. Therefore, for example, an impact absorbing material is provided in a collided portion to reduce the impact sound, or a decelerating material for applying a frictional load or the like to the cam or the reciprocating member in the area in which the acceleration torque is exerted on the camshaft is provided so that the acceleration torque can be reduced. However, when the impact absorbent material or the decelerating material is provided to the apparatus, a configuration of the apparatus becomes complicated. In addition, it is necessary to consider a time degradation of the impact absorbent material or the decelerating material. Moreover, when the decelerating material is provided to the apparatus, the decelerating material is set up to apply the load to the cam or the reciprocating member in such a direction that the load torque is exerted on the camshaft even when the acceleration torque is exerted on the camshaft, so that the average load torque increases, and thus a power consumption also increases.